• An infant with severe combined immunodeficiency syndrome (SCIDS) secondary to adenosine deaminase deficiency had pneumonitis and combined infection with respiratory syncytial virus (RSV) and parainfluenza virus type 3 (PIV3). Four separate courses of ribavirin were delivered by small-particle aerosol. The PIV3 disappeared during the first course, and RSV disappeared after the fourth course on the 58th hospital day. Neither virus returned during profound immunosuppression for bone marrow transplantation. Secretory antibody to both viruses was found and may have assisted in recovery. Strains of RSV from the 9th, 15th, 29th, and 55th hospital days showed similar sensitivities to ribavirin in vitro. Ribavirin can be a useful drug in the treatment of respiratory viral infections in patients with SCIDS.